Document how to write a blog post

[rlaverde]

[goanpeca]

@CAM-Gerlach, do you think this is worth adding?

[CAM-Gerlach]

The revised theme has full documentation on the technical aspects but a short higher-level guide might be nice. I can tackle it after I port the site (as much as possible despite being almost a complete rewrite under the hood, I kept the high-level API the same). Not a huge priority since the existing contributors are familiar but would be good to have.

[conradolandia]

Fixed with #43